There are now several technologies available for non-invasive imaging of the internal structure of the human body. These technologies include computerized tomography (CT), positron emission tomography (PET), and magnetic resonance imaging (MRI). These various broad approaches have advantages and disadvantages for imaging any particular portion or function of the body and variations on each technique have been found useful for particular applications or for imaging of particular bodily functions.
Imaging directed toward determining regional brain function can be conducted under present technology by several variations of these methods. The principle technique used to measure regional brain function is to measure regional cerebral blood flow (rCBF), which is most commonly measured at present by injection or inhalation of an inert diffusable tracer which can be followed by radionuclide or other detector systems. Recently introduced tomographic techniques give decidedly better information. CT, utilizing stable xenon as a tracer, PET scanning, and single photon emission computerized tomography (SPECT) are all tomographic techiques for brain imaging that have both particular advantages and particular limitations. PET is a quite powerful imaging device that can measure rCBF, regional cerebral blood volume (rCBV), and oxygen consumption and extraction, thereby giving a rather complete picture of cerebral metabolism which can be imaged regionally. Certain isotopes are available which also allow for regional glucose metabolism and neurotransmitter function to be measured. However the isotopes for use in PET scanning tend to be short lived and must often be produced by a nearby dedicated cyclotron with facilities for nuclear chemistry. Therefore the application of PET technology in clinical environments has been somewhat limited. SPECT does allow for tomographic evaluation of rCBF without dependence on a nearby cyclotron for isotopes, but the resulting image has relatively poor spatial resolution. Serial transmission CT using stable xenon as a tracer does provide rCBF determination with appropriate definition and has good spatial resolution. While such CT scanning can be widely used clinically, since access to a CT scanner is the principle need, it is not strictly a tracer method since the xenon must be given to the patient in pharmacologic doses that themselves effect brain metabolism.
Therefore all existing methodologies of measuring cerebral function have significant limitations. A hypothetical ideal method would be one that is not invasive, is rapid, can be tomographic in any desired plane, does not require tracers or pharmacologic agents, does not require radiation, and should be relatively inexpensive, so that it can be performed on readily available imaging equipment. The present invention is intended to satisfy many of these objectives.
Magnetic resonance imaging is most typically directed toward imaging of protons. Proton MRI has been very successful in defining anatomy and flow in blood vessels although as yet it has not yielded any data for studying tissue metabolism and perfusion. MRI equipment suitable for use in proton imaging can also be used to image sodium, in its most abundant naturally occurring isotope .sup.23 Na. .sup.23 Na is second only to protons from the standpoint of both natural abundance in the body and magnetic resonance imaging characteristics. It has been known before that sodium can be imaged using MRI techniques, but there have not been significant proposals for functional brain imaging with sodium scanning.